Production of aluminium hydroxide coated glass microspheres

ABSTRACT

A process for the coating of glass microspheres with an aluminium hydroxide, which involves the hydrothermal treatment of a suspension of complexed Al 3+ -ions with glass microspheres. The aluminium hydroxide coated glass microspheres can be applied in the production of light weight glass-reinforced plastic materials, e.g., for use in mass transportation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention provides a method for the coating of glassmicrospheres with an aluminium hydroxide.

2. Background Art

Applications for aluminium hydroxide coated glass microspheres are forexample in the production of light weight glass-reinforced plasticcomposite materials for use in mass transportation.

Aluminium hydroxide is well-established in the plastics industry as ahalogen-free filler which is used to impart fire retardant and smokesuppressing properties to synthetic thermosetting resin systems.Moreover, the trend towards stricter requirements on the burningbehavior of plastics means an increasing demand for aluminium hydroxideswhich can be incorporated in plastics at ever higher filling levels. Inglass-reinforced unsaturated polyester composites for example it is notuncommon for the aluminium hydroxide content of the composite materialto be >50 wt. %.

In mass transportation for instance, it is possible to achieve thestrict fire requirements on composite materials by using aluminiumhydroxide alone as fire retardant filler. At the same time, however, thelong term trend in composite materials is towards lighter weightcomposites which reduce energy costs. In the railways, for example, lessweight means an increased pay-load for high speed trains and less wearand tear on the tracks.

Aluminium hydroxide (Al(OH)₃) has a density of 2.42 g/cm³ which isconsiderably higher than the ca 1.1 g/cm³ of a synthetic resin. The useof aluminium hydroxide by itself therefore raises the density and henceoverall weight of the end-composite. In addition, the glass fibres usedfor structural composites contribute further to increased weight due tothe even higher density of ca 2.6 g/cm³.

Well-established in the art is the incorporation of hollow microspheresof aluminium silicate or borosilicate glass to decrease the overallweight of plastic composite materials and enhance their thermalinsulation characteristics. Such microspheres are available in particlesizes which are similar to those of aluminium hydroxide but they havedensities in the range of 0.1-1.0 g/cm³, so that their incorporation ineven relatively small amounts can provide a significant reduction inweight of the end composite. The problem when using hollow glassmicrospheres however is the need to maintain acceptable mechanicalproperties of the end composite. This is particularly important forstructural plastic composite materials. However, the well-rounded,smooth surfaces of the hollow glass microspheres militate against aneffective “bond” between the inorganic filler and the organic resin sothat the use of hollow of glass microspheres is generally accompanied bya deterioration in physical properties of the composite material.

BROAD DESCRIPTION OF THE INVENTION

An objective of the present invention therefore is to avoid the problemsassociated with the methods known in the art and to develop a method forapplying an aluminum hydroxide directly onto the surface of hollow glassmicrospheres.

The objective of the invention is achieved with the process according tothe invention, comprising the steps of

a) adding an organic complexing agent to an aqueous solution of anacidic aluminium salt

b) raising the pH of the solution to a value of ≦9,

c) adding a predetermined amount of said glass microspheres to saidsolution to form a suspension

d) hydrothermally treating said suspension in an oxidizing atmosphere atabout 150° C. to 350° C. and

e) separating the aluminium hydroxide coated glass microspheres from thesuspension.

It has been surprisingly discovered that a glass microsphere coatedfollowing the process of the invention provides uniform aluminiumhydroxide coating which remains firmly attached to the underlying glassmicrosphere surface during further processing thereby allowing theincorporation into synthetic resin of aluminium hydroxide coated glassmicrospheres with their associated fire retardant properties andproviding a substrate for using conventional additives which increasecompatibility and strengthen the “bond” between the inorganic andorganic phases.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a scanning electron micrograph of a typical appearance ofthe aluminium hydroxide coated glass microspheres.

DETAILED DESCRIPTION OF THE INVENTION

Preferred aluminium hydroxide applied is the boehmite type aluminiumhydroxide (AlOOH) which crystallizes out under the conditions used inthe process of the present invention and firmly adheres on the substratei.e. the glass microspheres.

Expediently, hollow microspheres of an aluminum silicate glass or a sodalime boro silicate glass are used. As preferred but non-limitingexamples of hollow glass microspheres the FG-200-7 glass microspheres(aluminium silicate glass) of the Fillite Company and the K20 ScotchliteGlass Bubbles of the 3M Company (boro silicate glass) can be mentioned.

The preferred starting solution is an aqueous solution of an acidicaluminium salt, expediently selected from the group consisting ofaluminium sulfate, aluminium nitrate and aluminum chloride in which thehydrated Al³⁺-ions are according to step a) of the invention initiallycomplexed and stabilized in solution by the addition of an organiccomplexing agent. Preferably organic acids, such as for example gluconicacid, tartaric acid, citric acid and oxalic acid are applied, usuallybefore raising the pH of the solution into the alkaline range.

According to step b) of the invention the pH of the solution is thenraised to a value of ≦9, preferably to about 11 by addition of aninorganic base, preferably selected from the group consisting of sodiumhydroxide, potassium hydroxide or of an aqueous solution of ammonia.

Subsequently according to step c) of the invention a pretermined amountof the glass microspheres is added to form a suspension. As a rule suchan amount is added so that a concentration of 50 to 100 g/l is reached.

The hydrothermal treatment according to step d) of the invention, isthen usually accomplished in an autoclave. Within this hydrothermaltreatment, performed at temperatures of about 150° C. to 350° C.,preferably at 250° C., the complex of the organic complexing agent withthe hydrated Al³⁺-ions oxidatively degrades. To support this oxidativedegradation, oxygen or an oxygen containing gas is preferably addedduring the hydrothermal reaction.

As the Al³⁺-complexes break down, the aluminium ions releasedcrystallize out of solution in the form of boehmite at the same elevatedtemperature. The crystalline boehmite is deposited directly on thesurfaces of the glass microspheres.

By varying the pH of the crystallizing solution, as a rule in the rangeof 10 to 12, the aluminium concentration, the nature of the complexingagent and/or the amount of oxygen used, influence can be exercised onthe particle size of the boehmite and hence thickness of the coating.

While all four parameters can affect boehmite particle size, only theamount of oxygen is of major significance, i.e., when oxygen is used. Ata predetermined pH, operating temperature and complexing agent,increasing the amount of oxygen used as a rule decreases the averagesize of the boehmite particle because of the faster initial release ofaluminium ions for crystallization. Conversely, decreasing the amount ofoxygen as a rule increases; the average size of the boehmite particles.

Separation of the aluminium hydroxide coated glass microspheres from thesuspension according to step e) of the invention can be effected bymethods known to those skilled in the art.

The coating can be varied in its thickness and adheres firmly to theunderlying surface during subsequent handling and processing insynthetic resins. The aluminium hydroxide coating imparts fire retardantproperties to the glass microspheres in addition to providing asubstrate for effective bridge-building between the filler and theorganic resin system.

The present invention will now be illustrated in detail with referenceto the following examples, which however are not to be interpreted aslimiting the scope of the invention.

EXAMPLE 1

A stock solution (21) of 0.45 M aluminium sulfate was prepared at roomtemperature by dissolving 600 g of Al₂(SO₄)₃.18H₂O in deionized water.To 500 ml of the stock solution in a 2 liter capacity glass beaker wasadded 500 ml of 1.5 M tartaric acid with agitation using a magneticstirrer. Into the vortex created by the stirrer was run 10 M NaOH untilthe pH of the resulting solution reached 11.2. To a 1 liter aliquot ofthis solution was added 80 g of glass microspheres (FG-200-7 grade;Fillite) and the whole transferred to a 3 liter capacity Inconelautoclave fitted with an internal coil for rapid heating and cooling.The glass microspheres had a density of ca. 0.5 g/cm³ and ranged inparticle size up to a maximum of ca. 150 μm.

Agitation of the autoclave contents was provided by a permanent magneticstirrer fitted with a six-blade turbine and operated at 330 rpm. Theautoclave was then closed and oxygen gas introduced at 25° C. to apartial pressure of 10×10⁵ Pa. The autoclave and contents were heated to250° C. and maintained at this temperature for 60 min.

On cooling back to conditions of atmospheric pressure and temperature,the suspension was removed from the autoclave and the solids filtered,washed with hot deionized water and dried at 110° C. The pH of theliquor filtrate was ca. 7.9 and was composed primarily of sodium sulfate(ca. 45 g/l sulfate) and the sodium salts of the organic carbondegradation products.

The dried product was boehmite coated glass microspheres and weighed100.1 g. The density of the aluminium hydroxide composite hollow glassmicrospheres was 1.00 g/cm³. X-ray diffraction confirmed that thecrystalline aluminium hydroxide coating was boehmite. The increase inaverage particle size indicated a boehmite layer thickness of ca 1 μm.Scanning electron microscope and EDAX analyses indicated that theboehmite is deposited uniformly over the microsphere surfaces.

The boehmite coating is highly polycrystalline in nature with theboehmite crystals intergrown and randomly oriented on the surfaces ofthe microsphere surfaces. The adherence of the boehmite to theunderlying microsphere surfaces was strong enough to survive a heattreatment at 1000° C. for 60 min in which the boehmite was calcined toaluminium oxide of density ca. 3.95 g/cm³.

EXAMPLE 2

The experimental procedure of Example 1 was repeated with the differencethat the system was seeded with borosilicate hollow glass microspheres(K20, scotchlite glass bubbles, 3M) which had a density of 0.20 g/cm³and ranged in particle size up to a maximum of ca. 90 μm. The driedproduct was boehmite coated glass microspheres and weighed 100.2 g. Thedensity of these aluminium hydroxide composite hollow glass microsphereswas 0.76 g/cm³. Particle size analysis indicated that the thickness ofthe boehmite coating was of the order of 0.1-1.0 μm.

What is claimed is:
 1. Process for the coating of glass microsphereswith an aluminum hydroxide comprising the steps of a) adding an organiccomplexing agent to an aqueous solution of an acidic aluminium salt b)raising the pH of the solution to a value of ≦9, c) after step b),adding an amount of said glass microspheres to said solution to form asuspension d) hydrothermally treating said suspension at about 150° C.to 350° C. and e) separating the aluminium hydroxide coated glassmicrospheres from the suspension.
 2. Process according to claim 1characterized in that the aluminum hydroxide applied is a boehmite-typealuminium hydroxide.
 3. Process according to claim 2 characterized inthat hollow microspheres of an aluminium silicate glass or a borosilicate glass are used.
 4. Process according to claim 3 characterizedin that the acidic aluminium salt is selected from the group consistingof aluminium sulfate, aluminium nitrate and aluminum chloride. 5.Process according to claim 4 characterized in that the organiccomplexing agent is an organic acid.
 6. Process according to claim 5characterized in that the organic acid is selected from the groupconsisting of gluconic acid, tartatric acid, citric acid and oxalicacid.
 7. Process according to claim 6 characterized in that raising thepH in step b) is effected by adding to said solution an inorganic base.8. Process according to claim 7 characterized in that the inorganic baseis selected from the group consisting of sodium hydroxide, potassiumhydroxide or of an aqueous solution of ammonia.
 9. Process according toclaim 7 characterized in that the glass microspheres are added in suchan amount to said solution that a concentration of 50 to 100 g/l isreached.
 10. Process according to claim 8 characterized in that thehydrothermal treatment in step d) is effected in the presence of oxygenor an oxygen containing gas.
 11. Aluminium hydroxide coated glassmicrospheres which have been produced by the process of claim
 1. 12.Process according to claim 1, wherein hollow microspheres of analuminium silicate glass or a boro silicate glass are used.
 13. Processaccording to claim 1, wherein the acidic aluminium salt is selected fromthe group consisting of aluminium sulfate, aluminium nitrate andaluminium chloride.
 14. Process according to claim 1, wherein theorganic complexing agent is an organic acid.
 15. Process according toclaim 14, wherein the organic acid is selected from the group consistingof gluconic acid, tartatric acid, citric acid and oxalic acid. 16.Process according to claim 1, wherein raising the pH in step (b) iseffected by adding an inorganic base to said solution.
 17. Processaccording to claim 16, wherein the inorganic base is selected from thegroup consisting of sodium hydroxide, potassium hydroxide and an aqueoussolution of ammonia.
 18. Process according to claim 1, wherein the glassmicrospheres are added in such an amount to said solution that aconcentration of 50 to 100 g/l is reached.
 19. Process according toclaim 1, wherein the hydrothermal treatment in step (d) is effected inthe presence of oxygen or an oxygen containing gas.